I would say something "obvious" in as matter-of-fact a way as I could which doesn't "explain" (since it is not the other person's business) but which at least acknowledges a question/concern has been broached:

"Those are just old scars." (or "that is just an old scar") and shake your head slightly in a "no"/dismissive fashion. Use your body language (shrugging too) to help make the other person realize that they have asked about something you don't wish to talk about.

"Yes, I hurt awhile back." (maybe add, "but I'm fine right now"); "agreeing" with them that you see what they see should stop additional questions as those would be truly nosey.

"Nothing much" or "Nothing worth talking about" "Nothing I want to talk about" (in response to "What happened?") and then change the subject to/back to something more neutral and off of you.